A solder used for joining electronic parts generally contains a solder alloy and a flux. The flux contains a modified rosin, a resin component composed of a synthetic resin or the like, and an activator, and further a solvent component and other additives, as needed. As the activator, halogen activators containing organic halogen compounds are known. For example, Patent Literature 1 discloses a flux containing, as an activator, a halogen compound in which halogen atoms such as bromine and chlorine are introduced into an organic compound by covalent bonds. Further, Patent Literature 2 discloses a flux containing an iodine-containing carboxyl compound as an activator. It is known that such a halogen compound can improve solder wettability by removing the oxide film on the surface of the solder or preventing reoxidation, and further reducing the surface tension of the solder.
However, it is difficult for the flux containing such a halogen compound activator to exert the aforementioned effects immediately after soldering, and it is also difficult to maintain the effects. Therefore, it is difficult to sufficiently improve the solder wettability in a general soldering step, which has been a problem.